1. Technical Field of the Invention
The present invention relates to a common mode filter, and particularly relates to a new common mode filter that secures a passage of an ultra high-speed differential signal propagated through an ultra high-speed differential transmission line, and attenuates a common mode signal.
2. Description of Related Art
In recent years, high-definition video contents such as “HDTV: high definition television” and “Blue-ray Disc” become widespread, and in order to transmit a vast amount of digital data for supporting such contents, high-speed serial transmission has been used.
In the high-speed serial transmission, a voltage amplitude needs to be reduced for shortening a rise time, and therefore this transmission easily receives an influence of a noise. Therefore, in order to hardly receive the influence of the noise, a differential transmission of a signal is generally used.
In a differential transmission system, smaller amplitude is secured for accelerating a transmission speed and realizing proper power saving by simultaneously transmitting positive phase and opposite phase differential signals to each of a pair of two lines, and a common mode signal such as an outside noise is canceled.
However, in the differential transmission system, a function of canceling the common mode signal such as an outside noise is insufficient, and therefore a common mode chalk coil is inserted into a differential transmission line, for preventing its adverse influence.
Conventionally, although not shown, it is generally known that in such a kind of common mode chalk coil, there is a magnetic core having two conducting wires wound thereon, with the same number of turns. FIG. 16 shows its circuit view.
In the common mode chalk coil with this structure, currents of differential signals flowing through the two conducting wires offset each other with opposite phases, thus generating no magnetic flux, and impedance of two conducting wires can be maintained to be low. Therefore, the differential signals can be easily passed.
Meanwhile, in a case of the common mode signal, currents flow through the two conducting wires, with the same phases, thus generating the magnetic flux in the magnetic body, and the impedance of the two conducting wires becomes high, to make it difficult to pass the signals. Therefore, the common mode signal is attenuated.
Japanese Patent Laid Open Publication No. 2000-58343(patent document 1) discloses a common mode chalk coil for differential transmission line, which corresponds to the structure of FIG. 16 as described above.
According to the patent document 1, two coil conductors are wrapped around a toroidal core, and the toroidal core is housed in a package case made of resin. The package case consists of a case and a lid. Ground conductors are formed by plating on outer surfaces of an external wall, a bottom wall, and the lid of the case. Insulating films are formed on the ground conductors. Terminal plates are bonded respectively to the insulating films, and the ends of the coil conductors are soldered to the terminal plates, to thereby make a characteristic impedance correspond to a transmission line so as to prevent signal reflection.
(Patent Document 1)
    Japanese Patent Laid Open Publication No. 2000-53343